feeds: use git-src-full to allow Git versioning

Both $(AUTORELEASE) and $(PKG_SRC_VERSION) (from luci.git) use the Git
log to determine releases and package timestamps.

Feeds are shallow cloned by default, resulting in an incomplete Git log
and therefore different local package versions than offered upstream.

This commits sets the default feeds to use `src-git-full` to solve that.

Signed-off-by: Paul Spooren <mail@aparcar.org>
Signed-off-by: Josef Schlehofer <josef.schlehofer@nic.cz>
[apply it for Turris OS feeds.conf; modify compile_pkgs and
documentation in generate_common]